The present invention relates to a processing apparatus, a processing method and a diamond tool, and in particular, to a processing apparatus, a processing method and a diamond tool which are suitable for processing a forming die for forming optical elements.
For processing an optical transfer surface of a forming die for forming high precision optical elements such as an objective lens of an optical pickup device, for example, there has been used an R-cutting tool made of mono-crystal diamond which has a nose radius of a face angle of about 0.1–1.5 mm and has a face angle of about 40–60°. When an optical transfer surface form is composed of a single surface expressed by a general aspheric surface equation, for example, there has been used a superfine processing machine, which has made it possible to obtain high precision optical transfer surface through only cutting operations. On the other hand, when conducting generating processing for the optical transfer surface having a structure of more fine forms, there is used an R-cutting tool having more fine nose radius, as is described in Patent Document 1.
(Patent Document 1)
TOKKAI No. 2003-62707
By using an R-cutting tool having a fine nose radius described in the Patent Document 1, it has become possible to generate an optical transfer surface which is more precise. In the field of optical pickup devices in recent years, however, there is a demand for conducting recording and/or reproducing of high density information by using a violet laser, and there is a requirement to bring the optical transfer surface form close to the ideal state to improve optical characteristics of optical elements used for optical pickup devices, responding to the demand mentioned above. However, there is a limit for the R-cutting tool stated above to generate precision optical transfer surface, because of the structure of the R-cutting tool. The reason for the foregoing will be explained as follows.
FIG. 1 is a sectional view of a forming die for forming an optical element, the forming die being cut by the use of a conventional R-cutting tool, and in this case, it is assumed that ring-shaped grooves for forming a ring-shaped zonal lens represented by a diffractive lens are generated on the optical transfer surface through cutting. R-cutting tool 3 has a cutting face that is composed of straight-line-shaped first edge portion 3a, straight-line-shaped second edge portion 3b and circular-arc-shaped third edge portion 3c. 
Die material 1 representing an article to be processed is cylindrical and is driven to rotate on rotation axis (optical axis of an optical element formed by a forming die (which is also called an optical axis of a forming die) 2. It is assumed that base optical transfer surface 1a is formed on die material 1 in advance corresponding to the optical aspheric surface. Under this condition, diamond tool 3 is given feeding in the optical axis direction (Z-axis direction) corresponding to a ring-shaped zonal shape, and is given feeding in the radial direction (X-axis direction) towards a center portion from an outer circumference side of metal mold material 1, thus, die material 1 is subjected to cutting operations. As a result, the metal mold material 1 is cut by diamond tool 3, and cylindrical surface 1b, ring-shaped optical transfer surface 1c and R curved surface 1d that connects the cylindrical surface 1b and ring-shaped optical transfer surface 1c are formed.
However, since an optical element in design is to be formed by a forming die wherein cylindrical surface 1b and ring-shaped optical transfer surface 1c are directly connected, if a forming die generated by the conventional processing method is used, there is a fear that design values are not satisfied by optical characteristics of the optical element. To be more concrete, R curved surface 1d which connects the cylindrical surface 1b to the ring-shaped optical transfer surface 1c is formed when a nose form of R-cutting tool 3 is transferred, and therefore, a form of a corner portion of a diffractive groove is different from a form in design, thereby, an optical path difference of diffracted light which should be generated originally fails to be generated. Therefore, there is a problem that diffraction efficiency of the optical element is lowered.
Further, on the optical element onto which R curved surface 1d that connects the cylindrical surface 1b to the ring-shaped optical transfer surface 1c is transferred and formed, a light flux which has entered a portion corresponding to the R curved surface 1d scatters without emerging as designed, which results in a decline of transmission factor of the optical element. To avoid the problem of this kind, there is an idea to sharpen a tip of the R-cutting tool (for example, cone-point cutting tool), which, however, causes another problem that a tip of the R-cutting tool tends to be broken by a stress in the course of processing, and surface roughness of the ring-shaped optical transfer surface 1c is worsened.